The present invention relates to a method and device for controlling the amount of exhaust gas recirculation (hereinafter abbreviated to "EGR" when applicable) in an internal combustion engine of an automobile or the like.
In a modern internal combustion engine, an electronic device such as a microcomputer is used to control the amount of EGR. In general, the amount of EGR is set by the operating position of needle valve in an EGR passage, which is in turn determined by various operating conditions of the engine. Particularly, the effective diameter of the EGR passage changes with the movement of the needle valve, and the amount of EGR is changed in proportion to the effective diameter. Therefore, EGR control can be achieved by feeding the operating position of the needle valve back and differencing it with the target position which is defined according to the desired amount of EGR.
The general arrangement and operation of a conventional device of this type will be described with reference to FIGS. 1 and 2. Further details of such a device are described in U.S. Pat. No. 4,257,382.
In FIG. 1, reference numeral 10 designates an EGR passage communicating the exhaust pipe of an engine (not shown) with the engine's intake manifold; 20, a needle valve provided in the EGR passage; 30, a position sensor for detecting the operating position of the needle valve 20; 40, a negative pressure motor or actuator, having a spring 41 and a diaphragm 42, for driving the needle valve 20 with a negative air pressure; 50, a solenoid valve including a valve 52 for opening the operating negative pressure chamber of the negative pressure motor 40 to the atmosphere and a solenoid 51 for operating the valve 52; 60, a solenoid valve composed of a valve 62 for opening the operating negative pressure chamber to a negative pressure source (for instance, the intake manifold) and a solenoid 61 for operating the valve 62; and 70, a control device implemented with a microcomputer. More specifically, the control device 70 includes a target value setting unit 71 for setting a target value a for the operating position of the needle valve 20 according to operating parameters of the engine, such as the engine speed and the temperature of the coolant, and a driving unit 72 for driving the negative pressure motor through the solenoid valves 50 and 60 according the positional deviation c between the target value a and the actual position b of the needle valve 20 which is detected by the position sensor 30. FIG. 1 shows in a sectional view the EGR passage 10 and the negative pressure motor.
In the device thus constructed, as the needle valve 20 is moved upwardly in FIG. 1, the effective diameter of the EGR passage 10 is increased, and as the needle valve 20 is moved downwardly, the effective diameter is decreased. Therefore, the effective diameter of the EGR passage is determined from the position b of the needle valve 20, and the EGR amount is determined from the effective diameter. That is, a desired amount of EGR can be obtained by setting the position b of the needle valve to a desired value. The aforementioned setting unit 71 is operated to set the position b of the needle value to a desired value.
A general description of the control of the needle valve 20 will be made with reference to a timing chart shown in FIG. 2.
In FIG. 2, a indicates the above-described target value; b, the aforementioned actual position; d, the open and closed states of the valve 62; and e, the open and closed states of the valve 52.
For instance, when the target value a is increased to open the EGR passage with the operation of the engine at the time instant T.sub.1, the positional deviation c is increased in the positive direction. In this case, the driving unit operates in response to the increase of the positional deviation c to open the valve 62 and to close the valve 52. As a result, the operating negative pressure chamber of the negative pressure motor 40 is opened through the valve 62 to the negative pressure source. Accordingly, the diaphragm 42 is pulled by the operating negative pressure against the force of the spring 41, while the needle valve 20 opens the EGR passage in assocation with the movement of the diaphragm 42. At the same time, the actual position b of the needle valve 20 is moved upwardly by this operation. The actual position reaches the target value a soon, and the positional deviation c is eliminated. Thereupon, the driving unit 72 operates to close both of the valves 52 and 62, as a result of which the operating negative pressure is held in the negative pressure chamber of the negative pressure motor 40 so that the actual position b of the needle valve 20 is maintained unchanged.
When the target value a is decreased to close the EGR passage at the time instant T.sub.2, the positional deviation c is increased in the negative direction. In this case, the driving unit 72 operates in response to the decrease of the target value a to open the valve 52 and to close the valve 62. As a result, the negative pressure chamber of the negative pressure motor 40 is opened to the atmosphere. Accordingly, the diaphragm is pushed downwardly by the force of the spring 41 while the needle valve 20 is also moved in the direction of closing the EGR passage 10. At the same time, the actual position b of the needle valve 20 is moved downwardly. The actual position b soon reaches the target value a, and the positional deviation c is eliminated. Thereupon, the driving unit 72 closes both of the valves 52 and 62, as a result of which the operating negative pressure is held in the operating negative pressure chamber of the motor 40 and the actual position b of the needle valve 20 is maintained unchanged. Thus, the position of the needle valve 20 is controlled to a value suitable for the operation of the internal combustion engine with the amount of EGR set to a suitable value.
It should be noted that the above general description of the needle valve position control disregards all the delays of operation which are involved in various parts of the control system, that is, the description has been made on the premise that the control system is ideal. In practice, the control system involves various delays of operation. For instance, the solenoid valve 50 or 60 takes a certain period of time to accomplish its opening or closing operation after it receives the drive signal, and the negative pressure motor 40 starts its driving operation a predetermined period of time after the valve has been closed or opened. FIG. 3 is a timing chart showing such delays of operation in the control system. In FIG. 3, f indicates a drive signal for opening and closing the solenoid valve 60; g, a drive signal for opening and closing the solenoid valve 50; and b, the actual position of the needle valve. FIG. 3 shows the delays which occur until the results of the drive signals f and g outputted by the control device 70 are fed back. As shown in FIG. 3, the feedback system includes times delays t.sub.1, t.sub.2, t.sub.3 and t.sub.4. For instance, for the time t.sub.2 after the "open" state of the drive signal f of the solenoid valve 60 has been changed to the "closed" state, the actual position b continues to move upwardly, and for the time t.sub.4 after the "open" state of the drive signal g of the solenoid valve 50 has been changed to the "closed" state, the actual position b continues to move downwardly. Since it takes a certain period of time for the control device 70 to perform its processing operation, a sampling period required for the control device 70 to sample the actual position b has a finite value. The delay due to the sampling period increases the aforementioned delay times t.sub.1, t.sub.2, t.sub.3 and t.sub.4, thus causing a so-called "limit cycle phenomenon" in which the feed-back actual position varies around the target position with a certain period. The limit cycle phenomenon results in positional "hunting".
This hunting phenomenon will be described with reference to the timing chart of FIG. 4. In FIG. 4, h indicates the sampling period T.sub.s of the actual position b of the control device 70. The other waveforms are the same as shown in FIGS. 2 and 3. It is assumed that the target value a increases. In this case, in synchronization with the sampling time, the drive signal f has the "open" state, and the actual position b moves upwardly, soon reaching the target value a. Since the sampling period T.sub.s is not zero, the drive signal f is maintained in the "open" state until the next sampling time after the actual position b has reached the target position a. At this next sampling time, the drive signal f will be in the "closed" state while the drive signal g will be in the "open" state. However, since the driving system incurs delays as described, for a certain period of time after the sampling time, the actual position b continues to move upwardly, as a result of which an overshoot occurs, as indicated at P.sub.1 in FIG. 4. After the delay times have passed, the actual position b starts moving downwardly. In this operation, as in the upward movement, an undershoot occurs, as indicated at P.sub.2 in FIG. 4. Subsequently, the overshoot and the understood are repeated.
Accordingly, an object of the present invention is to eliminate the position hunting phenomenon due to the delays of operation in the position control system and the finite sampling period. More specifically, an object of the invention is to provide an exhaust gas recirculation control method in which no hunting phenomenon is present and the position control is stable.